I added Guar gum and xanthane gum when I made ice cream, but 15% of it still melted before I put the ice cream in the fridge.
What ingredients can I use to prevent my ice cream from melting this fast?
Hydrocolloids like Guar and Xanthan gum don't prevent the ice cream from melting, rather they change the texture (more like jello). There are other ice cream additives. Instead, I'd recommend optimizing the physics before enlisting chemical help.
What happens is that the ice cream batter mixture warms up during the churning process and loses consistency (melts). The best defence against melting ice cream is working in lower temperatures:
You should be able to get great results using these simple steps, however, if it still looks like a losing battle, you can also try pre-cooling your churing bowl with dry ice. (if you throw the dry ice in the ice cream batter, you may get a tangy ice cream).
Afterwards, put the remainder of the dry ice in a container next to your machine and cover them both loosely with a tarp (keep the place ventilated) to keep the air around the ice cream batter and machine cold.